Catalytic steam reforming technology has been widely applied for synthesis gas production from hydrocarbon containing feedstock such as natural gas. Conventional steam methane reformers (SMR) employ tubular reactors packed with reforming catalysts in the form of pellets or structured catalyst packing. The tubular reactors are vertically mounted in parallel arrangement in a furnace. Since the steam reforming process is highly endothermic, external heating sources are required. Burners installed within the furnace housing combust natural gas or some other fuel to support endothermic reactions within catalyst filled tubes. Heat released from combustion reactions is transferred by radiation and convection to tubular reactor outer wall, then by conduction from the outer wall to the inner wall, and then by conduction and convection to the reaction mixture in the tubular reactor interior. A portion of the heat absorbed by the tubular reactor is utilized to bring natural gas and steam feeds from their feed temperature in a range of about 500° C. to about 550° C. to reaction temperature in a range of from about 650° C. to about 950° C. to achieve desired hydrocarbon conversion. Improving heat flux from tubular reactor outer environment to inner environment is a critical step to increase reactor efficiency.
Typical reforming catalysts in particulate form contain nickel as the active metal and alumina as the support material. The catalyst particles can be in the form of ring, cylindrical pellet, or of spherical shape. The catalytic endothermic reforming reactions occur on the catalyst particle exterior surface, as well as within the pores accessible to the reactants. The heat absorbed by the tubular reactor, conducts through the tube wall into the interior to support endothermic reactions. Both convective and conductive heat transfer mechanisms are in play inside the reactor tube. The lower thermal conductivity of catalyst particles affects heat available for endothermic reactions in the reactor interior. The upper limits on gas velocity to minimize pressure drop and/or prevent catalyst particle fluidization affect the heat transfer rate from the tube wall to the catalyst particles, on whose surface the endothermic catalytic reactions occur and where heat is needed. The tube wall temperature typically ranges from about 850° C. to about 1095° C. Such higher temperatures cause considerable expansion of tubes. Since the catalyst particles have a lower coefficient of thermal expansion, the potential exists for considerable slumping of the catalyst particles upon reactor heat up. This may cause suboptimal reactor performance due to inadequate heat transfer, higher pressure drop, and increased diffusional resistances. The potential also exists for catalyst attrition due to crushing forces when the tube contracts. Non-uniform reforming activity along the length of the tube can cause hot spots raising the tube wall temperature above safe value. Attempts have been made to address these problems by providing support structures to hold the catalyst particles in position or dividing the catalyst bed within the tube into multiple beds with support structures in between, however these add complexities and can result in undesirable higher pressure drop.
Structured catalyst inserts with metal substrates of different shapes have been proposed, for example U.S. patents such as U.S. Pat. Nos. 4,402,871; 4,162,993; 4,350,617; 7,968,208 disclose metal substrates of different shapes as catalyst carriers for gas cleaning applications. These can be formed starting with a metal substrate such as foil or mesh, and engineered into a particular shape. An interfacial layer containing one or more active metals or alloys can be deposited on the engineered shape to provide catalytic functionality. An intermediate interfacial layer may be deposited between the metal substrate and the catalyst containing interfacial layer to enhance structural integrity of thus formed outer catalyst containing interfacial layer. In some instances the proposed structured catalyst can be formed as a single element, and in some instances assembled from multiple elements. However, structured catalyst inserts for use in conventional SMR reformer tubes have their own design requirements, such as higher heat flux, turbulent gas mixing, lower pressure drop, and high reforming capacity. U.S. Pat. No. 7,320,778 B2 and U.S. Pat. No. 7,906,079 B2 disclose stackable structural catalysts with a central rod as support component for use in reforming reactors. The catalytic elements are formed as radial monoliths having corrugated metal leaves or fins to facilitate catalytic reaction, and the central rod provides structural support to the radial monoliths. U.S. Pat. No. 7,871,579 B2 developed the radially expandable catalyst inserts made of corrugated metal sheets for tubular reactors in order to maintain the gap between the orifice opening and the tube inside wall and adjust for tube creep. U.S. Pat. No. 5,599,509 discloses a metal catalyst converter in the honeycomb body created by alternately winding flat sheet and corrugated sheet. The corrugated sheets are disclosed to contain a plurality of slit matrix extended in a direction perpendicular to the exhaust gas direction in order to improve gas communications between each layer of corrugated metal sheet. Most of these works applied high temperature stable metal sheets and configured at least one corrugated metal substrate to form desirable geometry for maximizing catalytic activity and heat transfer while minimizing pressure drop. However, it is difficult to implement these corrugated metal substrates in small diameter reactors with high aspect ratios because the restrictions of tubing dimension limit the manufacturability of corrugated metal sheet based catalyst inserts and their surface area availability.
Gas hourly space velocity (GHSV) determined by dividing the volumetric flow rate of reactants at standard temperature and pressure of 0° C. and 1 atm by the amount of reforming catalysts (i.e. active metals with ceramic oxide carriers) in the reactor is commonly used to describe reactor reforming capacity. Tubular reactors in SMR plants using pellet catalysts typically operate in the range of about of 10,000 to 150,000 cm3/(hr g-cat). These tubular reactors are typically made of high temperature stable metal alloys such as Inconel 625 or 800 HT with an inside diameter (ID) in the range of about 4 inches to about 10 inches, and a length in the range of about 20 feet to about 40 feet. Thus, the ratio of reactor length to inside diameter also referred to as reactor aspect ratio ranges from about 24 to about 120. Depending on the geometry of the ceramic supports, the available surface area for catalyst coating is in the range of 500 to 1000 m2/m3. Typical particulate reforming catalysts contains Ni as the active metal on aluminum oxide support. Thickness of catalyst coating on the ceramic supports is generally less than 200 micrometer. These known catalyst particles have a lower coefficient of thermal expansion as well as a lower thermal conductivity than the metal tubes containing them. Another important factor that commonly used to describe reactor reforming capacity of a tubular reactor is the heat transfer rate. Surface heat flux is referred to as the rate of heat energy transfer through the reactor walls for a given tube surface area, while the volumetric heat flux is referred to the rate of heat energy transfer through the reactor walls for a given tube interior volume. Surface heat flux of typical full-size SMR reactor tubes at operational temperature ranges from about 100 to 200 kW/m2 and volumetric heat flux ranges from about 1000 to 8000 kW/m3.
Smaller diameter catalytic reactors can offer several advantages, for example improving heat transfer from external heat source to reaction mixture in the tube, enhancing tube life-time by reducing thermal gradients, reducing metal material use, and being applicable for compact steam reformer systems. Therefore, there are many works on exploring small diameter catalytic reactors. For example, U.S. Pat. No. 5,932,141 discloses a process of syngas production in series of tubular reactors with small diameter tube (e.g., 35 mm inside diameter) by applying a thin film of steam reforming catalyst on the inner wall of the tubular reactor. A different approach is described by U.S. Patent No. 20100178219 which discloses a highly heat integrated steam reformer/combustor assembly (or so-called ‘catalytic heat exchanger’) by coating the reforming catalysts and combustion catalysts on tubing walls.
To achieve similar production capacity as SMR plants, small diameter tubular reactors require a plurality of tubing components in series (high aspect) and much higher space velocity. It is challenging for direct catalyst coating on tubing wall to achieve high hydrocarbon conversion due to limitation of catalytic surface area and coating delamination at high operating temperature and large temperature gradient across tube wall.
The method of the invention overcomes the deficiencies of the prior art and allows efficient use of high aspect ratio catalytic reactors with tube inside diameter (ID) ranging from about 0.2 inches to about 2 inches. The method of the invention utilizes a metal foil having 3-dimensional surface features, rolled into a structure that serves as a catalyst carrier. Although the high temperature metal foils such as FeCralloy have been used in catalytic reactors in the past, they have not been usefully employed in high aspect ratio reactors because of the difficulty in establishing a stable supporting structure with adequate open gas channels. The catalyst inserts, structured monoliths of the present invention can be configured with a single layer metal sheet stamped with a plurality of peaks and grooves. Peaks on metal sheets serve as geometry support to prevent structure deformation and create multiples of open channels for reactant gas flow, while grooves on metal sheets provide open windows for gas communications between each layer of the monoliths. Different stamped patterns on metal sheet form designed peaks and grooves for various dimension of reactor tubing with inside diameter ranging from about 0.2 inch to about 2 inches. These metal monoliths with porosity from about 50% to about 85% significantly reduce the pressure drop in high aspect ratio reactor and provide sufficient catalytic surface area in steam methane reforming process. The present invention allows for the efficient use of metal foil comprising plurality of designed patterns on its surface, rolled into a multi-layer spiral shaped like a compact foil cylinder and catalyzed to serve as a catalyst insert in a high aspect ratio reactor. The designed patterns establish open gas channels between each of the rolled layers. This unique geometry accelerates gas mixing and the large surface area of metal foil provides a high catalytic active surface area.
With the present invention a novel reforming reactor configuration is also proposed. The reforming reactor utilizes one or more metallic tubular elements coupled together to form a reaction volume having an entrance for reactants at one end and an exit at the opposite end for reaction products. The reaction volume contains a plurality of catalyst inserts also referred to as catalyzed metal structures. The catalyzed metal structures can undergo thermal expansion when operating at elevated temperature without experiencing crushing forces from the reactor wall.